About 100 patients with Juvenile Arthritis will be typed for the HLA-DR antigen DRwl-DRw7 and WIa8. Since many of these will be patients that have not been studied before, a complete typing for HLA-A,B,C and for HLA-D will be performed. Fifteen large families including several members with definite adult type rheumatoid arthritis will be investigated. The HLA typing will be as complete as possible. These families will also be tested for rheumatoid factors, anti-nuclear factors and Epstein-Dan virus antibodies including the antibody to the rheumatoid arthritis nuclear antigen (RNA). Genotyped family members related to patients with rheumatoid arthritis will be used as subjects for investigation of the immune response in vitro. In a related project the HLA-D antigens in Black patients and of American Indian patients with rheumatoid arthritis will be investigated.